348 PHYSIOLOGY 



muscle in the body. Pinching the foot, instead of causing it to be drawn up 

 now causes the legs, arms and back to be rigidly extended. The extension 

 is not a co-ordinated act, but is associated with strong contraction of the 

 flexors, the final position of the limbs being determined by the preponderating 

 strength of the extensor muscles. The real meaning of this condition is seen 

 if, in a spinal mammal, the extensor muscles be connected with a lever and 

 the flexor muscles cut. On exciting the flexor reflex by pricking the foot, 

 there is instantaneous relaxation of the extensor muscles. A small dose of 

 strychnine is now given, insufficient to cause general convulsions. It is now 

 found that on pricking the foot the extensor muscles respond, not with 

 inhibition, but with a contraction. Strychnine acts by abolishing the in- 

 hibitory side of every co-ordinated act and converting the process of 

 inhibition into one of excitation. Co-ordination therefore becomes an im- 

 possibility, and stimulation of any spot excites contractions not only of the 

 appropriate muscles but also of the antagonists of these muscles, the direction 

 of the resulting movement being determined simply by the relative strength 

 of the two sets of muscles. 



The same effect is produced by tetanus toxin, and, since the action of this 

 toxin may be confined in its early stages to one limb, it is possible to show 

 the abolition of the inhibitor side of the reflexes in this one limb while the 

 limb of the other side reacts normally to the stimulus. The same abolition of 

 inhibition is found whether the response be excited by stimulation of the 

 skin or by voluntary excitation from the cortex of the brain. Thus in the 

 monkey, on stimulating the cortex, opening of the mouth may be excited 

 from all the spots marked " <[ " in the diagram, closure being only obtained 

 from those spots marked " = " (Fig. 173). Under the influence of the 

 tetanus toxin excitation of every one of the spots, whether " < " or " =," 

 causes closure of the jaw. It is impossible for a patient under these circum- 

 stances to open his mouth, because every willed impulse for opening in- 

 nervates at the same time the stronger masseter muscles and effectively 

 closes the mouth. 



